I wondered the same thing. I don't know the tech issues, but you could always forward from your Hotmail account to a Gmail or Yahoo account, set your reply from address in the Gmail account to the Hotmail account and still have your Hotmail on your BB.

P.S. Astro Boy could kick MegaMan's butt!

Or just forward it to your BB email account. Best option would be to just drop hotmail, its crap and leveraged to only work well on Windows with IE. Google mail is better, works with all platforms and works great with the BB.